The present invention relates to an energizing and deenergizing device for a d.c. motor, and particularly to a brushless d.c. motor having a rotary spindle whose coefficient of friction is very small.
In a case where a brushless d.c. motor is used to drive a turntable of a video disc player or the like, the turntable may continue to rotate for a long period of time even if the power supply thereto is removed when the reproduction of music is over. This phenomenon causes the exchange of discs to be difficult, and, therefore, it has been common to design the motor energizing and deenergizing device such that the motor and hence the turntable is stopped upon termination of the power supply to the motor when reproduction is ended.
FIG. 1 shows an example of the conventional motor driving and stopping device for a video disc player. In FIG. 1, a brushless motor is connected to a driving circuit 1. A d.c. generator 3 is mechanically coupled to a rotational shaft (not shown) of the motor 2 so that a d.c. voltage, the level of which is in proportion to the rotational speed of the motor 2, is induced at an output of the d.c. generator 3. A comparator 4 is connected to the output of the d.c. generator 3 and functions to compare the output voltage of the d.c. generator 3 with a predetermined reference voltage Vr. An output terminal of the comparator 4 is connected to the driving circuit 1.
With the motor driving and stopping device constructed as above, when a high level rotation instruction signal such as shown in FIG. 2a is supplied to the driving circuit 1, the latter responds thereto to control the brushless motor 2 to rotate in a predetermined direction, at a predetermined rotational speed. At this moment, since the output voltage of the d.c. generator 3, the level of which is proportional to the speed of the d.c. motor 3, is larger than the reference voltage Vr, the output of the comparator 4 is at a low level. Then, when the rotation instruction signal to the drive circuit 1 is terminated and the input level to the drive circuit 1 becomes low, the drive circuit 1 drives the motor 2 in a manner so as to urge rotation in the reverse direction. Therefore, a rotational torque is developed in a direction opposite to the predetermined direction, and consequently the rotational speed of the motor 2 in the predetermined direction is gradually decreased. In reaction, the output voltage of the d.c. generator 3 is also lowered as shown in FIG. 2b, and when it becomes smaller than the reference voltage Vr, the output of the comparator 4 assumes a high level as shown in FIG. 2c. With this high level output of the comparator, the drive circuit 1 functions to stop the driving of the motor 2, resulting in no torque being generated by the latter. Therefore, the motor 2 is rotated freely only by the inertia of the turntable (not shown). It should be noted that the drive circuit 1 functions to drive the motor 2 in the predetermined direction so long as the rotation instruction signal is supplied, even if the output level of the comparator 4 is high.
In the conventional motor drive/stop device constructed as above, in order to shorten the time duration from the commencement of free rotation of the motor to complete stoppage, it is sufficient to set the reference voltage at a low value. However, since the frictional coefficient of the motor spindle depends upon the motor used even if the reference voltage is set at a given low level, the setting of the reference voltage per se is difficult and there may be problems such that, depending upon the motor to be used, it may be impossible to completely stop the motor within a desired short time duration.